Fix attempt for unicode bug #1137 (v3)

New pull request because Github just doesn't get rebases. Previous PRs: #1179, #1772 Issue: #1137

Uses existing "native_str()" function that [has been added](https://github.com/pypa/pip/commit/7c652cb07ecc32313466cc3ce01b4bba0cca5eb7) in the meantime.